<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>border</title>
   
</head>
<body>
    <div clss="border">0.5px边框</div>
    <style>
        .border {
            width: 200px;
            height: 200px;
            margin: 0 auto;
            border: 1px red solid; 
            border-bottom: 1px solid transparent;
            border-image: linear-gradient(to bottom,50%, red 50%) 0 0 100%/1px 0;
        }
    </style>
</body>
</html>